1. Field of the Invention
This invention relates to a liquid crystal compound useful as a material for an optical device, a liquid crystal medium and an optical device, and in particular, to a liquid crystal compound having a large dielectric anisotropy, a large optical anisotropy and good compatibility, and a liquid crystal medium with a wide temperature range of liquid crystal phase, a large dielectric anisotropy and a large optical anisotropy. This invention also relates to an optical device using the liquid crystal medium, and in particular, to an optical device that can be used in a wide temperature range and driven at a low voltage and can achieve a high-speed electro-optical response.
2. Description of Related Art
Liquid crystal display (LCD) devices utilizing liquid crystal compositions are widely used in displays of clocks, calculators, word processors and so on. These LCD devices utilize the optical anisotropy and the dielectric anisotropy, etc. of liquid crystal compounds. The operation modes of LCD devices mainly include phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and so on, which utilize one or more polarizers for display. Moreover, in recent years, more attentions have been paid to the mode where an electric field is applied to an optically isotropic liquid crystal phase to make the liquid crystal phase exhibit electrically controlled birefringence (Patent References 1-9 and Non-patent References 1-3).
Moreover, tunable filters, wavefront control devices, liquid-crystal lenses, aberration correction devices, aperture control devices and optical head devices, etc., which utilize the electrically controlled birefringence in a blue phase as one of optically isotropic liquid crystal phases, have been proposed (Patent References 10-12).
According to the driving mode, the LCD devices can be classified into passive matrix (PM) and active matrix (AM) types. The PM type is further classified into static type and multiplex type, etc., and the AM type is further classified into thin film transistor (TFT) type, metal insulator metal (MIM) type and so on.
Such LCD devices contain a liquid crystal composition with suitable properties. To improve the properties of an LCD device, the liquid crystal composition preferably has suitable properties. A liquid crystal compound as a component of a liquid crystal composition needs to have the following general properties: 1) high chemical stability and physical stability, 2) a high clear point (i.e., phase transition temperature from the liquid crystal phase to the isotropic phase), 3) a low lower-limit temperature of liquid crystal phase (i.e., nematic phase, cholesteric phase, smectic phase, optically isotropic liquid crystal phase like blue phase, etc.), 4) good compatibility with other liquid crystal compounds, 5) a suitable dielectric anisotropy, and 6) a suitable optical anisotropy.
Particularly, for an optically isotropic liquid crystal phase, a liquid crystal compound having a large dielectric anisotropy and a large optical anisotropy is preferred from the viewpoint of lowering the driving voltage.
When a liquid crystal composition containing a liquid crystal compound having stable chemical and physical properties as described in (1) is used in an LCD device, the voltage holding ratio is improved.
Further, a liquid crystal composition containing a liquid crystal compound with a high clear point or a low lower-limit temperature of liquid crystal phase as described in (2) and (3) can have a wide temperature range of nematic phase or optically isotropic liquid crystal phase, and thus can be used in a display device in a wide temperature range. A liquid crystal compound is generally mixed with a number of other liquid crystal compounds to prepare a liquid crystal composition, so as to exhibit properties difficult to develop by a single compound. Thus, a liquid crystal compound used in an LCD device preferably has good compatibility with other liquid crystal compounds as described in (4). In recent years, LCD devices with superior properties, especially display performances, for example, contrast, display capacity and response time, are especially required in the industry. In addition, regarding the liquid crystal material used, a liquid crystal composition with a low driving voltage is required. Furthermore, in order to drive an optical device driven in an optically isotropic liquid crystal phase with a low voltage, a liquid crystal compound with a large dielectric anisotropy and a large optical anisotropy is preferred.
It has been reported that chlorobenzene derivatives analogous to the compound of this invention have large dielectric anisotropy and large optical anisotropy (Patent Reference 13), but a compound having a chlorophenyl moiety and the good properties of the compound according to this invention have not been reported.